A PON system is an optical communication system that performs optical splitting in a P2MP (Point to Multi Point) connection mode, with no electrical power. The PON system includes a station side device; a PON line of a tree structure composed of a single-core optical fiber network which is split with the station side device at the top; and a plurality of customer side devices connected to the ends of the split optical fibers, respectively (see, for example, Patent Literatures 1 to 3).
In the PON system, NRZ (Non-Return to Zero) optical signals obtained by directly or externally modulating a light source such as a semiconductor laser are transmitted, by which predetermined information is transmitted and received.
In such a PON system, downstream frames composed of optical signals transmitted from the station side device are transmitted to the customer side devices in broadcast form. Each customer side device receives a downstream frame destined therefor (also including the case of multicast) by referring to a logical link identifier (e.g., an LLID in the case of a PON specified in IEEE Std 802.3™-2008. In the following, it may be representatively described as LLID.) which is included in a preamble of the downstream frame and which is notified by the station side device upon discovery, and discards other downstream frames.
On the other hand, for upstream frames composed of optical signals sent out to the PON line from the customer side devices, in order to prevent collisions, the station side device performs multi-point access control in a time-division manner for each LLID. Therefore, the station side device receives the upstream optical signals from the customer side devices in a burst manner.